Cargo power drive unit

ABSTRACT

An apparatus ( 100 ) for conveying cargo is provided, comprising a support base ( 105 ) and a drive assembly ( 110 ) pivotably mounted to the support base ( 105 ) for vertical movement. The drive assembly ( 110 ) comprises a roller ( 120 ) and a first electronically controlled electric motor ( 125 ) for rotating the roller ( 120 ). A lift assembly has a second electronically controlled motor ( 155 ) for moving the drive assembly ( 110 ) relative to the support base ( 105 ) from a retracted position to an extended position with the roller ( 120 ) in driving abutment against the cargo. An electronic controller ( 195 ) controls the first ( 125 ) and second ( 155 ) motors.

FIELD OF THE INVENTION

[0001] The present invention relates to cargo conveying systems for usein moving cargo on a vehicle such as an aircraft. The present inventionhas particular applicability in roller drive systems in which a rolleris urged upwardly against cargo to be conveyed when the system isactivated and retracted when the system is deactivated.

BACKGROUND ART

[0002] Roller assemblies which are removably mounted on carriersattached to the floor of a vehicle are widely used for conveying cargowithin such vehicles. These roller assemblies or “actuators” areintended for use in systems which involve the transport of commercialand military cargo containers commonly used in the air cargo, groundcargo (trucking), rail cargo and shipboard cargo industries. Theactuator installs into a cargo transport vehicle, and provides motiveforce and holding (i.e., braking) for the containers within the vehicle.

[0003] The actuator, commonly called a power drive unit or powered driveunit (PDU), works in conjunction with external support equipment tofacilitate the loading and unloading of the cargo containers into andout of the vehicle. The PDU typically incorporates a fixed frame orsupport base rigidly attached to the floor or floor structure of thecargo vehicle. A moving drive assembly is attached to the support base.The drive assembly includes drive roller or rollers for contacting thecargo containers and providing motive force to them by means of thefrictional coefficient between an elastomeric roller surface and thecontainer bottom, and a motor for rotating the drive roller. Cargocontainer bottoms are generally smooth, metallic coverings attached to asquare or rectangular frame structure; however, they may also be made ofnon-metallic material such as wood or reinforced plastic.

[0004] The PDU also incorporates a lift mechanism which raises the driveassembly from its lowered position into abutment against the cargocontainer bottom, as by rotating cams on a common camshaft againstreaction points or bearings in the support base. The lift mechanismprovides the vertical force to the drive roller, which in turn providesthe frictional force to drive the container. The lift mechanism also canbe engaged to hold the container in place when the drive roller is notturning, to prevent the unintended motion of containers in the vehicle.

[0005] When drive and/or hold commands are removed from the PDU, the PDUdrive assembly lowers to its retracted position. The cargo containersare typically supported by a plurality of free turning cylindricalrollers, ball transfer units or caster assemblies, which arecollectively referred to as the conveyance hardware. When the PDU isretracted, the containers are free to move on the conveyance hardware,and are typically held in position during vehicle motion by latches,guides and other restraint hardware.

[0006] Typical PDUs are described in U.S. Pat. No. 3,690,440, U.S. Pat.No. 3,737,022 and U.S. Pat. No. 5,547,069, wherein a single motor isused to rotate an elastomeric drive roller and to power a lift mechanismwhich raises a drive assembly carrying the roller into abutment againsta cargo container bottom. Another PDU is described in U.S. Pat. No.5,803,234 to Podanski et al., wherein separate motors are provided forthe drive and lift functions. In Podanski's PDU, the drive motor is astandard AC induction motor, and the lift motor is a brush-type DCmotor.

[0007] Prior art PDU's can involve complex mechanisms which are bothcostly and difficult to repair or replace. However, regardless of theirdegree of complexity, prior art PDU's are limited in their operationalflexibility under varying conditions, such as use in different vehiclesor for different types of cargo containers or loading. As a result, theyare not easily adaptable to different applications requiring changes indrive torque and/or lift force, and are subject to overheating andoverloading due, for example, to “scrubbing” of the drive roller underparked or jammed containers.

SUMMARY OF THE INVENTION

[0008] An advantage of the present invention is a PDU whose drive andlift functions are totally independent of each other and electronicallyprogrammable, thereby enabling flexibility in setting and changingoperating parameters such as vehicle interface functions, logicfunctions, drive torque and lift force, and enabling the inventive PDUto be adapted to be used in a variety of different vehicle applications.

[0009] According to the present invention, the foregoing and otheradvantages are achieved in part by an apparatus for conveying cargocomprising a support base and a drive assembly pivotably mounted to thesupport base for limited vertical movement substantially normal to thelongitudinal axis of the support base, the drive assembly comprising aroller and a first electronically controlled electric motor for rotatingthe roller. A lift assembly has a second electronically controlledelectric motor for moving the drive assembly relative to the supportbase from a retracted position substantially within the support base toan extended position with the roller in driving abutment against thecargo, and an electronic controller controls the first and secondmotors.

[0010] Additional advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only the preferred embodiment of thepresent invention is shore and described, simply by way of illustrationof the best mode contemplated for carrying out the present invention.The advantages of the invention may be realized and obtained asparticularly pointed out in the appended claims. As will be realized,the present invention is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Reference is made to the attached drawings, wherein elementshaving the same reference numeral designations represent like elementsthroughout, and wherein:

[0012]FIG. 1 is a perspective view of an apparatus in accordance with anembodiment of the present invention.

[0013]FIG. 2A is a top view of the apparatus of FIG. 1.

[0014]FIG. 2B is a side view of the apparatus of FIG. 1.

[0015]FIG. 3 is a cross-sectional view taken through line A-A of FIG.2B.

[0016]FIG. 4 is a perspective view of an apparatus in accordance with anembodiment of the present invention.

[0017]FIGS. 5A and 5B are graphical representions of a power consumptioncurve used in an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

[0018] Conventional PDU's are mechanically complex and lack flexibilityunder varying operating conditions or vehicles. The present inventionaddresses and solves these problems of conventional PDU's.

[0019] The PDU of the present invention includes two separateelectronically controlled electric motors, one for the drive functionand one for the lift function. These motors can both be permanent magnetalternating current (PMAC) motors, commonly referred to as “brushlessDC” motors. The motors have their own control electronics, which areintegrated into the PDU's electronics module. The dual electronicallycontrolled motor arrangement of the present invention providesindependent, fully controllable power to both the drive and liftfunctions of the PDU. For example, the maximum drive torque and themaximum lift force of the PDU can be set and changed independently bythe control software of the PDU, thereby providing maximum flexibilityfor using the actuator in various cargo applications. Furthermore,unlike prior art actuators, the present invention does not retract andthen lift upon change of drive direction; the drive roller stays inconstant contact with the container when the drive direction isreversed, thereby avoiding unwanted cargo movement when changing drivedirection. Moreover, the use of electronically controlled motors in theinventive actuator improves upon the state of the art by providingincreased motive force, reduced power consumption and reduced weightcompared to prior art actuators.

[0020] An embodiment of the present invention will now be described withreference to FIGS. 1-4. A PDU 100 according to the present inventioncomprises a support base 105 of a metal such as aluminum, to which ismounted drive means, such as a drive assembly 110 pivotably mounted viapivot pins 115 for limited vertical movement substantially normal to thelongitudinal axis L of support base 105. Drive assembly 110 comprises adrive roller 120 having an elastomeric outer surface, and means forrotating roller 120, such as an electronically controlled electricmotor; e.g., a conventional PMAC motor 125 for rotating roller 120 viaplanetary gear assembly 130, pinion gear 135, idler gear 140 and outputgear 145 (see FIG. 3).

[0021] The inventive PDU also incorporates lift means, such as liftassembly 150, for raising the drive means from its lowered position, asby rotating cams on a common camshaft against reaction points orbearings in the support base. The lift means provides vertical force tothe drive roller, which in turn provides the frictional force to drivethe container. More specifically, lift assembly 150 comprises anelectronically controlled electric motor such as a conventional PMACmotor 155. As shown in FIGS. 3 and 4, lift motor 155 transmits powerthrough a planetary gear assembly 160, a lift cable driver 165, liftcables 170 and a lift pulley 175 to a cam shaft 180 which carries a pairof cams 185 at either end. Cams 185 react against bearings 190 supportedby lift reaction pins 190 a in support base 105 to move drive assembly110 relative to support base 105 around pivot pin 115 from a retractedposition substantially within support base 105 (as shown in FIGS. 1-4)to an extended position wherein roller 120 is in driving abutmentagainst the cargo (not shown). Lift motor 155 drives at approximately11,000 rpm as drive assembly 110 is raising, and then operates in astalled condition when drive roller 120 comes in contact with acontainer bottom, or when lift assembly 150 reaches its internal liftstop (not shown).

[0022] Lift assembly 150 incorporates tension cables 170 to transmittorque from lift motor 155 and gearing 160 to lift cams 185. In priorart actuators, the common means to provide torque to lift cams is to usegearing. The layout of the present invention is designed so that driveroller 120 is located between motors 125, 155 and lift cams 185. Thislayout provides optimum load distribution and minimizes the loadstransmitted into support base 105. Torque transmission from the liftgearing 160 to cams 185 is accomplished by one or more multi-strand,steel or composite cables or straps 170, which are pulled by driver 165at the output of lift gearing 160, and in turn pull cams 185 at a fixedradius. Cams 185 rotates against reaction bearings 190 in support base105, thereby providing both vertical motion and vertical force to driveroller 120. Since an independent lift motor 155 is used, lift assembly150 is designed to rotate in a single direction, regardless of thedirection of drive roller rotation. The cable lift mechanism of thepresent invention provides a lighter weight, lower cost solution whichcan be operated in an open environment, eliminating the need for theprior art's sealed gearing at the output of the lift mechanism.

[0023] PDU 100 also comprises control means for independentlycontrolling the lift means and the means for rotating the roller, suchas an electronic controller 195 for electronically controlling drivemotor 125 and lift motor 155 in a conventional manner as required by allelectronically controlled motors, and for performing logical, memory andother operations which will be described in detail hereinbelow.Electronic controller 195 also includes built-in-test (BIT) functions toverify the proper operation of the actuator prior to use. Electroniccontroller 195 includes an input/output port 200, such as a serial port,for receiving signals from an external controller (not shown) and forsending signals to the external controller or other external devices,such as a computer. Electronic controller 195 further comprises anon-volatile memory that is used to collect and retain operatinginformation of PDU 100, such as total operating time, total operatingcycles, or any other data available to electronic controller 195. Thisdata can be extracted via input/output port 200 for purposes of trackingreliability data or verifying usage.

[0024] Electronic controller 195 performs several critical functions. Itcontrols independent lift and drive motors 125, 155, vehicle interfaceand logic functions, as well as providing electromagnetic interferencecontrol. Conventional power control modules for both motors 125, 155 areincorporated into electronic controller 195, which in one embodiment ofthe present invention comprises ‘cool’ MOSFETs (metal oxidesemiconductor field effect transistors) for power switching. Thesedevices have advantageously low power dissipation, and are ideallysuited to application in a PDU. Electronic controller 195 can alsoutilize advanced IGBTs (integrated gate bipolar transistors) for othercritical switching functions. Each motor 125, 155 can be controlled byits own microprocessor within electronic controller 195 which storessoftware code, including the current and force limit settings for themotors, acceleration and deceleration rates, timeouts and delays, andany other necessary information. Electronic controller 195 can alsoinclude an EMI (electro-magnetic interference) filter to limit both theconducted EMI from PDU 100 as well as the EMI susceptibility of PDU 100to within industry standards.

[0025] PDU 100 further includes two separate non-contacting sensors 205which detect the presence of cargo containers above PDU 100. Sensors 205can be one or more of several types of non-contacting sensors, e.g.infrared, ultrasonic, Hall effect, etc. The sensor cargo presencesignals are received by electronic controller 195, which operates motors125, 155 only if it receives the cargo presence signal from at least oneof the sensors 205. Electronic controller 195 applies ‘OR’ logic toinsure that PDU 100's motors 125, 155 operate when one sensor 205experiences a failure (ON or OFF). The dual sensors provide addedprotection from contamination that might block one sensor 205 and renderPDU 100 inoperable. Alternatively, electronic controller 195 can receivea manual override signal from the external controller throughinput/output port 200, and in response will operate motors 125, 155regardless of whether the cargo presence signal is received from sensors205. This manual override mode is used for system check-out and forsensor fault override.

[0026] In addition to cargo sensors 205, temperature sensors 205 a areprovided for monitoring the temperature of motors 125, 155 and thetemperature of electronic controller 195. Electronic controller 195 alsomonitors motor operating parameters such as electrical current draw andspeed of motors 125, 155 in a conventional manner employing sensorsincorporated into motors 125, 155, such as rotor position sensors, whichare required for operation of motors 125, 155. Using sensors 205 a andits monitoring functions, electronic controller 195 provides internalself-protection against overheating, as well as protection against bothdrive motor and lift motor overloads. As discussed above, electroniccontroller 195 can provide feedback to the external controller throughinput/output port 200. Such feedback signals can include but are notlimited to container presence, drive motor speed, drive roller speed,drive or lift motor current or corresponding loads, thermal protectionstatus and a PDU identifier code.

[0027] Drive and lift motors 125, 155 and their associated gearing aredesigned to be installed into a center housing 210 of PDU 100 comprisinga single, continuous bore. Center housing 210 contains both drive andthe lift motors 125, 155, planetary lift gearing 160, planetary primarydrive gearing 130, inboard bearing support 215, outboard bearingsupports 220, 225 and end covers 230, 235. The advantages of thisconfiguration are the ease of manufacture of the housing bore 210, theease of assembly of components into a common bore, and the eliminationof shimming during assembly. Shimming is a time consuming and errorprone step in the assembly process which is avoided in this embodimentof the present invention. Shimming is eliminated in the inventiveapparatus, despite the numerous axial components in the bore, becauseinboard bearing support 215 not only supports the inboard bearings ofthe rotors of both drive and lift motors 125, 155, but also provides anaxial spring 215 a which compensates for the tolerance stack-up in thebore of center housing 210. The assembled components are compressedagainst spring 215 a and are retained by end covers 230, 235 therebyeliminating gaps between all the components in the bore of centerhousing 210.

[0028] Lift assembly 150 also includes a one-way clutch 240 between liftmotor 155 and lift gearing 160 to eliminate the impact loads which wouldnormally occur when drive assembly 110 retracts to the lowered position.During retraction, lift motor 155 accelerates to high speed; clutch 240allows motor 155 to ‘free-wheel’ to a stop after lift cams 185 contacttheir down direction stop (not shown).

[0029] The performance and operational availability of a PDU is directlyrelated to the size and durability of its drive roller(s). The size ofthe drive roller (based on its diameter and length) relative to theoverall ‘footprint’ or top view of the PDU is advantageously maximizedto insure good contact with the container, as well as long roller life.Drive roller 120 of PDU 100 of the present invention encompassesapproximately 23% of its support base 105's footprint (i.e., the lengthand width of support base 105), with a total surface area ofapproximately 48 square inches. By comparison, prior art PDUs used inair cargo have single or dual drive rollers that are about 12% to 17% ofthe footprint area, and total surface areas of 7 to 39 square inches.

[0030] The fixed support base of the inventive PDU 100 is designed tominimize weight and cost, yet still transfer the required loads fromdrive assembly 110 to the vehicle floor structure. This is accomplishedby the layout and load distribution described above, which transfers theloads imparted to drive roller 120 through pivot pins 115 and lift cams185 and into the corners of base 105 through reaction pins 190. Thisminimizes bending and deflection in support base 105, and allows base105 to be manufactured with aluminum sheet stampings or thin-walledextrusions.

[0031] Several exemplary actuation functions or “modes” of PDU 100 willnow be described. The “retracted mode” is the normal rest mode of PDU100, wherein external power is removed, no drive or hold command is sentfrom the external controller, or no container is present (i.e., no cargopresence signal is generated by sensors 205). Retract mode facilitatesthe manual movement of containers. since drive assembly 1 10 isretracted below the conveyance plane.

[0032] In the “lift and drive mode”, lift assembly 150 raises driveassembly 110 into contact with a container and drive roller 120 rotatesto move the container. A direction command from the external controllerand the cargo presence signal are required for this mode. Unlike typicalprior art actuators having a single motor for both lifting and driving,PDU 100 does not retract and then lift upon a change of drive direction.Rather, drive roller 120 stays in constant contact with the containerwhen the drive direction is reversed, because electronic controller 195causes drive motor 125 to reverse direction while maintaining liftassembly 150 in the extended position.

[0033] In the “hold mode”, lift assembly 150 raises drive assembly 110into contact with a container and drive roller 120 is restrained fromrotating by drive motor 125 and its associated drive gearing 135, 140,145. The hold function of the inventive PDU 100 does not require aseparate brake (e.g., a friction-type brake), because drive motor 125 islocked electronically by electronic controller 195 to prevent rotationof drive roller 120. This mode is intended to stop and hold containersin position, and can be used continuously when power is available to PDU100. The hold mode is selectable via a command from the externalcontroller such that the operator can choose to have PDU 100 hold orretract when the drive command is removed. For example, a pin ofinput/output port 200 can be connected to the vehicle ground as a signalto electronic controller 195 that the hold mode is desired. This can beapplied without operator input, such as when a cargo door of an airplaneis opened and the PDU system is turned on, all PDU's are placed in holdmode. The hold mode is typically maintained by the airplane's logiccontrollers when AC power is interrupted. For example, the hold mode canbe engaged by a relay in the airplane.

[0034] Unlike prior art PDUs, the actuator of the present invention hasthe ability to resume the container holding mode after AC power isremoved and subsequently re-applied without operator input. This isparticularly important in the air cargo industry, where AC power istypically supplied by a ground power cart, aircraft generators, or anaircraft auxiliary power unit (APU), and is subject to momentaryinterruption and re-application at any time. Prior art PDUs can losecontact with and control of the cargo container upon power interruption,because they retract immediately upon power interruption and require adrive command to be reapplied by the operator upon resumption of ACpower delivery to resume holding (since they typically have a commonlift and drive motor). In contrast, PDU 100 of the present inventionretracts upon interruption of AC power, but when power is reapplied,sensors 205 detect cargo above them, and electronic controller 195 seesthe hold signal, because the hold mode was engaged by the aircraftcontroller prior to power interruption, as discussed immediately above.In response to the cargo presence signal and the hold signal. electroniccontroller 195 automatically causes lift assembly 150 to move from theretracted to the extended position and drive assembly 110 to hold thecargo in place. A drive signal from the operator is not necessary.

[0035] In a further embodiment of the present invention, PDU 100incorporates a traction and scrubbing control function, featuringsensors and logic which limit unnecessary scrubbing of the drive rollerunder parked or jammed containers, thereby avoiding excessive driveroller wear and motor overheating. When a container reaches its intendedposition against a latch or another container, it is said to be parked.Most prior art PDUs will continue to drive against a parked containeruntil the drive command is removed, or until the PDU overheats. SomePDUs have incorporated ‘time-outs’ or container motion sensors to limitscrubbing under parked containers. However, time-outs do not allow theoperator the flexibility to work with various sizes of containers, andmotion sensors may not work well under wet or slow moving containers.

[0036] Electronic controller 195 of PDU 100 of the present inventionanalyzes the amount of torque applied to drive roller 120 (which isproportional to drive motor 125's current draw) and the motor 125's runtime to determine the instantaneous power consumption of drive motor125. More particularly, electronic controller 195 includes conventionalcircuitry for measuring the electrical current draw of drive motor 125and for monitoring the measured current draw. Electronic controller 195compares a power consumption curve stored in its memory to the monitoredcurrent draw, and shuts off drive motor 125 when its power consumptionexceeds this curve. Under high traction loads, the drive time is limitedto reduce drive roller scrubbing. Under lighter loads, as areencountered with lightweight or wet containers, drive motor 125 runs aslong as the drive command is applied. This logic reduces the likelihoodof ‘false trips’, and lets drive motor 125 run long enough to ‘squeegee’a wet drive roller and move wet containers.

[0037] In one embodiment of the present invention, scrubbing (i.e.,controlled roller wear) is accomplished by turning off drive motor 125when the accumulated energy delivered to drive roller 120 via drivemotor 125 exceeds a preset limit. The accumulated energy(E_(accumulated)) is calculated by summing the total dissipated energy(E_(dis)) sampled over time, Dissipated energy is energy (E_(in)) thatis being delivered to motor 125 in excess of a predetermined thresholdenergy value (E_(th)). Threshold and accumulated energy are programmableinto electronic controller 195 for each PDU application and are governedby the following equations:

E _(in) =∫P _(in) dt=∫(I _(s) ² *R _(m))dt   (1)

E _(th) ∫P _(th) dt=∫(I _(th) ² *R _(m))dt   (2)

E _(dis) =E _(in) −E _(th)   (3)

E _(accumulated) =ΣE _(dis) dn   (4)

[0038] where:

[0039] R_(m)=Motor resistance, normalized=1

[0040] I_(s)=Sampled current

[0041] P_(in)=Power into motor 125

[0042] I_(th)=Threshold value determined for each application

[0043] P_(th)=Threshold power.

[0044] Sample power consumption curves are depicted in FIGS. 5A and 5B.FIG. 5A shows PDU energy usage with a variable load at drive roller 120,and FIG. 5B shows accumulated energy with a variable load at driveroller 120.

[0045] The PDU of the present invention is designed to operate in anumber of different vehicle applications, with no changes to theactuator. It can be pre-programmed to work in several applications wherea different traction forces, lift forces or logic delays are required.Electronic controller 195 has sufficient memory to store multipleprograms for multiple PDU applications. As discussed above, feedbacksignals can also be provided to an external controller. Any one of aplurality of different external controllers can be used with a singleinventive PDU by using different electrical interface cables, such ascable 300 shown in FIG. 1. The interface cables attach to input/outputport 200 but have a distinct vehicle interface connector, such asconnector 300 a, based on the controller application. Circuitry internalto the particular electrical interface cable, such as circuitry 300 b,identifies the cable type to electronic controller 195 via input/outputport 200, and in response electronic controller 195 selects and usessoftware specific to that application stored in its memory. Thus, thesame PDU 100 can be installed in a different position or a differentvehicle type, and work properly without any changes to the unit.

[0046] The inventive PDU features two separate permanent magnetbrushless DC electric motors, one for the drive function and one for thelift function. The motors have their own control electronics, which areintegrated into a PDU electronics module. The drive motor is currentlimited to control the maximum available drive torque, which can bevaried for different cargo applications along with the maximum liftforce of the PDU, via control software. The dual motor arrangementprovides independent, fully controllable operation of both the drive andlift functions of the PDU of the present invention. This independenceand programmability provides maximum flexibility for using the inventivePDU in various applications.

[0047] The present invention can be practiced by employing conventionalmaterials. methodology and equipment. Accordingly, the details of suchmaterials, equipment and methodology are not set forth herein in detail.In the previous descriptions, numerous specific details are set forth,such as specific materials, structures, chemicals, processes, etc., inorder to provide a thorough understanding of the present invention.However, it should be recognized that the present invention can bepracticed without resorting to the details specifically set forth. Inother instances, well known processing structures have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

[0048] Only the preferred embodiment of the present invention and but afew examples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is capable ofchanges or modifications within the scope of the inventive concept asexpressed herein.

What is claimed is:
 1. An apparatus for conveying cargo, the apparatuscomprising: a support base; a drive assembly pivotably mounted to thesupport base for limited vertical movement substantially normal to thelongitudinal axis of the support base, the drive assembly comprising aroller and a first electronically controlled electric motor for rotatingthe roller; a lift assembly comprising a second electronicallycontrolled electric motor for moving the drive assembly relative to thesupport base from a retracted position substantially within the supportbase to an extended position with the roller in driving abutment againstthe cargo; and an electronic controller for controlling the first andsecond motors.
 2. The apparatus of claim 1, comprising a sensor fordetecting the presence of the cargo above the drive assembly and sendinga cargo presence signal to the electronic controller.
 3. The apparatusof claim 2, wherein the electronic controller is for operating themotors only if it receives the cargo presence signal.
 4. The apparatusof claim 1, comprising at least two sensors, each for detecting thepresence of the cargo above the drive assembly and respectively sendinga cargo presence signal to the electronic controller, wherein theelectronic controller is for operating the motors only if it receivesthe cargo presence signal from at least one of the sensors.
 5. Theapparatus of claim 3, comprising an input device for sending a manualoverride signal from a user to the electronic controller, wherein theelectronic controller is for operating the motors when it receives themanual override signal and does not process the cargo presence signal.6. The apparatus of claim 4, comprising an input device for sending amanual override signal from a user to the electronic controller, whereinthe electronic controller is for operating the motors when it receivesthe manual override signal and does not receive the cargo presencesignal.
 7. The apparatus of claim 2, wherein the sensor comprises anon-contacting sensor.
 8. The apparatus of claim 7, wherein the sensorcomprises a Hall effect sensor, an infrared sensor, or an ultrasonicsensor.
 9. The apparatus of claim 1, wherein the lift assembly comprisesa cam rotatably mounted in the drive assembly and rotated by the secondmotor for reaction against a bearing in the support base to provide thevertical movement to the drive assembly; wherein the drive roller isdisposed between the second motor and the cam.
 10. The apparatus ofclaim 9, wherein the bearing in the support base is supported by a pindisposed proximal to a corner of the support base.
 11. The apparatus ofclaim 10, wherein the support base comprises aluminum.
 12. The apparatusof claim 9, wherein the lift assembly further comprises a flexibletension cable connected to the second motor and the cam for transmittingtorque from the second motor to the cam.
 13. The apparatus of claim 12,wherein the flexible tension cable comprises a multi-strand steel cable.14. The apparatus of claim 12, wherein the flexible tension cablecomprises a composite cable or strap.
 15. The apparatus of claim 1,wherein the electronic controller is for controlling the first motor torotate the roller in a first direction or in a second direction oppositethe first direction; wherein when the lift assembly is in the extendedposition and the roller is rotating in the first direction, theelectronic controller is for causing the first motor to rotate theroller in the second direction while maintaining the lift assembly inthe extended position.
 16. The apparatus of claim 2, wherein theelectronic controller is for receiving a hold signal from an externalcontroller; and wherein the electronic controller is for causing thelift assembly to move from the retracted position to the extendedposition, and the first motor to hold the roller stationary, when theelectronic controller receives the hold signal and the cargo presencesignal.
 17. The apparatus of claim 1, comprising a circuit for measuringelectrical current draw to the first motor, wherein the electroniccontroller is for monitoring the current draw of the first motor, andfor removing power from the first motor when the current draw of thefirst motor exceeds a predetermined current draw for a predeterminedtime period.
 18. The apparatus of claim 17, wherein the electroniccontroller comprises a memory for storing a power consumption curverepresenting a plurality of current draw values and corresponding timeperiods, and the electronic controller is for calculating thepredetermined current draw and predetermined time period based on thepower consumption curve.
 19. The apparatus of claim 18, wherein thepower consumption curve is for limiting the time of operation of thefirst motor to reduce scrubbing of the roller against the cargo.
 20. Theapparatus of claim 1, wherein the drive assembly comprises a continuousbore, and the first and second motors are mounted coaxially inside thebore.
 21. The apparatus of claim 20, wherein the first and second motorseach comprise a shaft and an inboard bearing for supporting the shaft;and wherein the drive assembly further comprises an inboard bearingsupport for commonly supporting the inboard bearings of the first andsecond motors inside the bore.
 22. The apparatus of claim 21, whereinthe inboard bearing support comprises an axial spring for compensatingfor tolerance in the bore.
 23. The apparatus of claim 21, furthercomprising primary drive gearing attached to the first motor shaft andlift gearing attached to the second motor shaft, wherein the bore is forhousing the primary drive gearing and lift gearing.
 24. The apparatus ofclaim 1, wherein the apparatus has a footprint defined by the length andwidth of the support base, and the roller has a footprint defined by thediameter and length of the roller, wherein the footprint of the rolleris greater than 20 percent of the footprint of the apparatus.
 25. Theapparatus of claim 24, wherein the footprint of the roller is about 23percent of the footprint of the apparatus.
 26. The apparatus of claim16, wherein the electronic controller comprises an input/output port;wherein the apparatus is controllable by the external controller via theinput/output port; and wherein the electronic controller is for sendingfeedback signals to the external controller, the feedback signalsincluding at least one of an identifier code for the apparatus, thecargo presence signal, a first motor speed signal, a roller speedsignal, a first motor load signal, a second motor load signal, and athermal protection status signal.
 27. The apparatus of claim 1, whereinthe electronic controller comprises a non-volatile memory for storingoperating information relating to the apparatus.
 28. The apparatus ofclaim 27, wherein the operating information includes at least one oftotal operating time of the apparatus and total operating cycles. 29.The apparatus of claim 27, wherein the electronic controller comprisesan input/output port for downloading information from the non-volatilememory to an external computer.
 30. The apparatus of claim 1, whereinthe electronic controller is for providing control, programmable by auser, of at least one of first motor traction force, second motor liftforce, first and second motor speed, logic functions and electromagneticinterference control.
 31. The apparatus of claim 1, comprising firs, andsecond motor temperature sensors, each for sending a respective motortemperature signal to the electronic controller.
 32. The apparatus ofclaim 1, comprising: an input/output port of the electronic controller;and an interface cable for connecting to the input/output port and toone of a plurality of different external controllers for controlling theapparatus, the interface cable having circuitry for identifying the oneexternal controller to the electronic controller; wherein the electroniccontroller is for selecting and using software specific to the oneexternal controller responsive to the cable circuitry.
 33. The apparatusof claim 12, wherein the second motor is for moving the drive assemblyfrom the extended position to the retracted position, and the liftassembly comprises a one-way clutch between the second motor and theflexible tension cable for allowing the second motor to free-wheel afterthe second motor moves the drive assembly from the extended position tothe retracted position.
 34. An apparatus for conveying cargo, theapparatus comprising: a support base; drive means mounted to the supportbase for limited vertical movement substantially normal to thelongitudinal axis of the support base, the drive means comprising aroller; means for rotating the roller; lift means for moving the drivemeans relative to the support base from a retracted positionsubstantially within the support base to an extended position with theroller in driving abutment against the cargo; and control means forindependently electronically controlling the lift means and the meansfor rotating the roller.
 35. A method of conveying cargo, the methodcomprising: mounting a drive assembly having a roller for limitedvertical movement substantially normal to the longitudinal axis of asupport base; providing a first electronically controlled electric motorfor rotating the roller; moving the drive assembly relative to thesupport base from a retracted position substantially within the supportbase to an extended position with the roller in driving abutment againstthe cargo; providing a second electronically controlled electric motorfor moving the drive assembly; and electronically controlling the firstand second motors.
 36. The method of claim 35, comprising detecting thepresence of the cargo above the drive assembly and operating the motorsonly if the cargo is detected.
 37. The method of claim 35, comprising:providing at least two sensors, each for detecting the presence of thecargo above the drive assembly; and operating the motors only if cargois detected by at least one of the sensors.
 38. The method of claim 37,comprising sending a manual override signal from a user; and operatingthe motors when the manual override signal is sent and cargo presence isnot detected.
 39. The method of claim 35, comprising controlling thefirst motor to rotate the roller in a first direction or in a seconddirection opposite the first direction; and causing the first motor torotate the roller in the second direction while maintaining the driveassembly in the extended position when the drive assembly is in theextended position and the roller is rotating in the first direction. 40.The method of claim 36, comprising: receiving a hold signal from anexternal controller; and causing the drive assembly to move from theretracted position to the extended position, and the first motor to holdthe roller stationary when the hold signal is received and cargo isdetected.
 41. The method of claim 35, comprising: measuring electricalcurrent draw to the first motor; monitoring the current draw of thefirst motor; and removing power from the first motor when the currentdraw of the first motor exceeds a predetermined current draw for apredetermined time period.
 42. The method of claim 41, comprisingstoring a power consumption curve representing a plurality of currentdraw values and corresponding time periods, and calculating thepredetermined current draw and predetermined time period based on thepower consumption curve.
 43. The method of claim 42, wherein the powerconsumption curve is for limiting the time of operation of the firstmotor to reduce scrubbing of the roller against the cargo.
 44. Themethod of claim 35, comprising sending feedback signals to an externalcontroller, the feedback signals including at least one of an identifiercode for the apparatus, the cargo presence signal, a first motor speedsignal, a roller speed signal, a first motor load signal, a second motorload signal, and a thermal protection status signal.
 45. The method ofclaim 35, comprising providing control, programmable by a user, of atleast one of first motor traction force, second motor lift force, firstand second motor speed, logic functions and electromagnetic interferencecontrol.
 46. The apparatus of claim 1, wherein the first and secondmotors are permanent magnet alternating current motors.